A continuing need in the pressure-sensitive adhesive (PSA) art is achievement of better control over various mechanical and process properties so that adhesives can be "tailor-made" for specific, highly demanding end-use applications such as packaging, medical, and masking tapes. These applications require a proper balance of properties, and this balance changes with the particular end-use.
Among the earliest polymers to provide a reasonable balance of the properties required for satisfactory PSA performance were the natural and synthetic rubbers. However, these PSAs had poor aging properties, and the art quickly recognized the need to eliminate ethylenically unsaturated groups from the polymer backbone of rubber adhesives in order to improve their oxidative stability.
This became possible with the discovery of Ziegler-Natta (ZN) coordination catalyst systems. With the discovery of ZN catalysts, it became possible to polymerize .alpha.-olefin monomers to high molecular weight. The homopolymers of the C.sub.6 -C.sub.10 .alpha.-olefins were naturally tacky and therefore good candidates for PSAs since they also had low toxicity, good aging and favorable environmental stability (i.e., low oxidation). These homopolymers were chemically inert, resistant to plasticizer migration, and relatively inexpensive. However they had poor cohesive strength and therefore lacked the shear adhesion necessary for high performance PSAs.
Use of ZN catalysts to make homopolymers from .alpha.-olefin monomers, and copolymers from mixtures of .alpha.-olefin and non-conjugated polyene monomers is old in the art and is not the subject of this invention. ZN polymerizations are succinctly summarized in Odian, G., "Principles of Polymerization", Ch. 8.4 (Second Edition, J. Wiley & Sons, New York, 1981). For a more detailed discussion of the polymerization of .alpha.-olefins, see Boor, J., "Ziegler-Natta Catalysts and Polymerizations", Ch. 19 (Academic Press, NY, 1979).
Radiation curing has been frequently used to chemically crosslink the polymeric component of adhesives in attempts to increase the cohesive strength of coated adhesive films. The results have been unsatisfactory because competing crosslinking and degradation (chain scission) reactions occur simultaneously. Therefore in some PSA systems, especially those formulated from polymers containing propylene, radiation curing leads to a loss of cohesive strength and shear adhesion.
Adhesives derived primarily from C.sub.6 or larger .alpha.-olefins are well known in the art. U.S. Pat. No. 3,542,717 describes poly(.alpha.-olefin) adhesive compositions comprising mixtures of polyolefin copolymers derived from olefin monomers with different molecular weights. (i.e. .alpha.-olefins having 11-20 carbon atoms, and different .alpha.-olefins having 4-20 carbon atoms.) The resulting adhesive has high shear and peel adhesion at room temperature, but at high temperatures cohesive strength and shear adhesion are unacceptably low for most applications.
U.S. Pat. No. 3,635,755 describes PSA polyolefin compounds suitable for use as single component PSAs for surgical tapes that are substantially non-allergenic. Such adhesives can be made from homopolymers of the C.sub.6 to C.sub.11 .alpha.-olefins or from interpolymers of C.sub.2 to C.sub.16 .alpha.-olefins having an intrinsic viscosity of 1.5 to 7.0, a Williams' plasticity of 0.8 to 4.0, and an acetone/heptane soluble fraction of less than 25% by weight.
Tapes made from these adhesives have low shear adhesions that facilitate non-irritating removal from the skin. However, this low shear adhesion is unsuitable, for example, for masking or packaging tape applications.
U.S. Pat. Nos. 3,954,697 and 4,178,272 describe hot-melt adhesives derived from copolymers of propylene and C.sub.6 -C.sub.10 .alpha.-olefins. The '697 patent teaches that while C.sub.6 --C.sub.8 .alpha.-olefin homopolymers are permanently tacky, they have poor cohesive strength and consequently lack shear adhesion. Therefore the higher .alpha.-olefins must be copolymerized with propylene in order to meet "the critical property requirements for a pressure-sensitive adhesive material" (col. 1, line 37). The copolymer must contain more than 40 mole % propylene, especially if static shear is to exceed 1000 minutes. The '272 patent describes compositions of ZN catalyst system and process conditions useful in preparing the "tacky" copolymers required for PSAs. Also disclosed are stereo-regulating three-component ZN catalyst systems that produce high molecular weight, crystalline block copolymers unsuitable for PSA applications (col. 2, lines 39-65), but are stated to be useful in sealing bags, cartons, and as an adhesive for floor tiles.
This approach is further advanced in U.S. Pat. No. 4,288,358 which describes hot-melt adhesive compositions containing terpolymers of propylene, 1-butene1-pentene, and C.sub.6 -C.sub.10 .alpha.-olefins; tackifying resins; and plasticizing oils, in yet further efforts to simultaneously optimize the adhesive and process properties of these compositions.
While these references teach optimized processing conditions for preparation of hot melt adhesives that are readily coatable or extrudable at the melt temperature, these materials have poor PSA properties. In particular, they have poor adhesion under shear. To be useful, high temperature masking tapes used in the automotive industry must have good "shear adhesion". Attempts to improve these propylene containing poly(.alpha.-olefin) adhesives of the references by radiation crosslinking have led to a degradation of PSA properties, such as shear adhesion.
U.S. Pat. No. 2,933,480 describes two component coordination catalyst systems (ZN) capable of copolymerizing mixtures of .alpha.-olefins and non-conjugated diolefins to yield sulfur-vulcanizable elastomers described as "rubber-like" with iodine numbers of 3 to 50, and intrinsic viscosities of 0.5 to 5.0.
U.S. Pat. Nos. 4,340,705 and 4,551,503 describe 3-component ZN catalyst systems for the copolymerization of mixtures of .alpha.-olefins and non-conjugated .alpha.,.omega.-diolefins to yield sulfur vulcanizates having high molecular weight, high inherent viscosity (5 or greater), low gel (0-5%), and improved (higher) unsaturation which provide improved vulcanizate properties.
No utility as adhesives is taught or claimed for the vulcanizates described in any of the three patents referenced above.